“It is bad to lose muscle instead of fat, because muscles are the key players in body movement and function,” says Gerardo Miranda-Comas, MD, Assistant Professor of Rehabilitation Medicine, Icahn School of Medicine at Mount Sinai. “With loss of muscle mass, strength and endurance are affected negatively, leading to decreased functional performance.”

But your body shouldn’t naturally go for muscle first in weight loss – if you’re doing it right.

“In general, muscle is not lost before fat – it is very dependent on nutrition and activity volume,” Miranda-Comas says. “A person who is attempting to lose weight by not eating may lose weight in muscle first before fat.”

How does that happen? Well, the body likes to go for carbs (glucose) for energy first. If that’s not available, it goes for glycogen, which is glucose that’s been stored in the liver and muscles, says Dr. Miranda-Comas.

“Fat is also used for energy depending on duration of the physical activity, and an individual can train their body to use fat as the primary source of energy,” says Dr. Miranda-Comas. (Hello, ketosis—the basis of the keto diet.)

“A healthy diet is usually 45-65 per cent carbs, anywhere from 15-35 per cent protein, and 20-35 per cent fat,” says Wesley Delbridge, RDN, spokesperson for the Academy of Nutrition and Dietetics. “The point is having a balanced diet and not restricting any food very low.”

This, of course, depends on the diet you’re following. Some may be low-carb (like the keto diet), high-protein (like the Atkins diet), or low fat. But if you’re taking it to extremes, your body will start to feel it.

1. Your workout feels more strained

“You’ll notice less strength in the gym. The weight you used to be able to do for reps may decrease or you may not be able to get as many reps as you once did for each set,” says Spencer Nadolsky, D.O., author of The Fat Loss Prescription.